1. Field of the Invention
The present invention relates to a multimode semiconductor laser module, a wavelength detector, a wavelength stabilizer, and a Raman amplifier.
2. Related Background Art
There are semiconductor laser modules (abbreviated as LD modules hereinbelow) of two types: a single-mode LD module generating a single-mode laser beam and a multimode LD module generating a multimode laser beam.
A single-mode LD module is used, for example, to generate signal light in a Wavelength Division Multiplexing (WDM) communication system. In WDM communication, signal light with a narrow line width is required. For this reason, the output wavelength of a single-mode LD module has to be controlled to an order of 10 pm. An optical element called etalon, which utilizes optical interference, is used to satisfy this requirement.
Wavelength stabilization of a single-mode laser using an etalon is disclosed in Japanese Patent No. 2914748, U.S. Pat. No. 5,825,792, and European Patents Nos. EP 0 926 789 A2 and EP 0 883 216 A2. A single-mode laser beam is introduced into an etalon from a LD module. Even slight change in the wavelength of the laser beam greatly changes the intensity of light emitted from the etalon due to interference of the incident light into the etalon. Detecting the change in the light intensity using a photodiode allows slight change in the output wavelength of the LD module to be detected. The output wavelength of the LD module is controlled according to the output of the photodiode. Stabilization of the output wavelength is attempted in this way.